This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-139679, filed May 20, 1999, the entire contents of which are incorporated herein by reference.
This invention relates to a distance-measuring device used in an image pickup device, such as a camera for using a silver-salt film, a digital camera, or a video camera, and more particularly to a distance-measuring device for measuring subject distances at points on the shooting screen (so-called multi-autofocus, hereinafter referred to as multi-AF), characterized by a wide distance-measuring area, such as full-screen AF.
Today, it is becoming quite common for image pickup devices, such as cameras, to implement multi-AF. Cameras equipped with a distance-measuring device for measuring subject distances at three, five, or seven points on the shooting screen are commercially available in low-price models.
The multi-AF is one-dimensional multi-AF where distance-measuring areas are arranged on a straight line. Recently, there have been signs of the commercialization of two-dimensional multi-AF or area AF.
For example, a camera equipped with a distance-measuring device having an area AF function using as many as 45 distance-measuring areas 17 on the finder view 16 has been commercialized and put on the market.
In such conventional multi-AF, complex calculations, such as distance-measuring calculations, must be repeated as many times as the number of distance-measuring areas increases. To improve the time-lag, various inventions have been disclosed.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2-158705 has disclosed the technique for acquiring pieces of subject distance information in a first distance-measuring mode in which the distances to points on the subjects are measured roughly, selecting the subject presenting the shortest distance, and measuring the distance only to the selected subject with high accuracy in a second distance-measuring mode, thereby improving the time-lag.
Furthermore, Jpn. Pat. Appln. KOKAI Publication No. 63-131019 has disclosed a technique which is based on the assumption that, in active AF, the closest main subject is present at the place where the amount of light reflected from the projected light is the greatest and which omits distance-measuring calculations for parts where the amount of reflected light is small, thereby improving the time-lag.
Since all the conventional AF methods have used active AF, they have improved the time-lag remarkably. When they attempt to perform full-screen AF or the like, however, a set of projecting elements and a set of light-receiving elements cannot avoid becoming very large, which is an obstacle to putting the device to practical use.
In contrast, passive AF would miniaturize the light-receiving elements much more than active AF, causing no obstacle to putting the device to practical use. Thus, passive AF is more suitable for wide-range multi-AF, such as full-screen AF, than active AF.
In this connection, Jpn. Pat. Appln. KOKAI Publication No. 62-103615 has disclosed the technique for performing rough correlation operation on distance-measuring areas, selecting one of the distance-measuring areas on the basis of the result of the operation, and then performing high-accuracy correlation operation on only the selected distance-measuring area, thereby improving the time lag by passive AF.
The rough correlation operation is performed by thinning out sensor data items, such as using every other sensor data item in calculations, but can never be omitted. Thus, although active AF has a higher efficiency of time-lag measures than passive AF, both types of AF produce the same effect.
A recently proposed solution to the problem of which of passive AF and active AF is more suitable for wide-range multi-AF, such as full-screen AF, is a distance-measuring method using hybrid AF. In hybrid AF, which is now in use, a steady-state light removing circuit for removing steady-state light is provided for each light-receiving element in a passive sensor. A passive operation is performed, when the steady-state light removing function is disabled, whereas an active operation is performed when the steady-state light removing function is enabled. Jpn. Pat. Appln. KOKAI Publication No. 10-336921 has disclosed such a steady-state light removing circuit. Products using hybrid AF are already available on the market.
To perform wide-range multi-AF, such as full-screen AF, time-lag measures are essential. For this reason, various devices have been thought out to avoid using high-speed, expensive CPUs and microcomputers at the sacrifice of cost. One of principal devices divides the process of measuring the distance in two: the first half of the process for pre-distance measurement and the second half of the process for actual distance measurement.
The purpose of the pre-distance measurement is to measure the distance roughly in a short time and estimate the position of the main subject, whereas the purpose of the actual distance measurement is to limit time-consuming high-accuracy distance measurement to the necessary-minimum subjects on the basis of the result of the pre-distance measurement in the preceding process. Although the process of short-time pre-distance measurement increases, the time required to measure the distances to the subjects excluded is eliminated, which helps shorten the entire distance-measuring time.
More specifically, in one type of pre-distance measurement, light is projected onto subjects and the position of the main subject is estimated on the basis of the amount of reflected light. It is common practice to set a subject with a value equal or close to the largest amount of reflected light as a candidate for the main subject.
In the estimating method, however, there can be too many candidates for the main subject, depending on the type of subjects. This can make the measures against time-lag insufficient, which is a drawback in the prior art and therefore a problem to be solved.
It is, accordingly, an object of the present invention to provide not only a high-accuracy, low-cost distance-measuring device and a high-accuracy distance-measuring method which enable only the subject distances at the necessary minimum positions to be measured, regardless of the type of subjects, have less time-lag, enable faster operation, and assure a high reliability of the result of distance measurement, but also a camera using the device and method.
According to a first aspect of the present invention, there is provided a camera comprising: first distance-measuring means for measuring a subject distance on the basis of a subject image signal produced by a pair of integration-type light-receiving sensors receiving the light from subjects; second distance-measuring means for measuring the subject distance on the basis of an image signal from which steady-state light removing means has removed the steady-state light component from the subject image signal, while projecting means is projecting light onto the subjects; subject select means for causing the second distance-measuring means to operate for a specific time and, according to the image signal obtained from that operation, selecting a subject whose distance is to be measured; mode setting means for setting the operation mode of the camera; and upper limit setting means for setting an upper limit on the number of subjects the subject select means selects according to the operation mode set by the mode setting means.
According to a second aspect of the present invention, there is provided a distance-measuring device for a camera comprising: a projecting device for projecting light onto subjects; a light-receiving sensor for receiving the light from the subjects; a distance-measuring area setting circuit which operates the projecting device and sets a distance-measuring area according to the output of the light-receiving sensor; a mode setting circuit capable of setting more than one shooting mode; and a control circuit for controlling the number of distance-measuring areas set at the distance-measuring area setting circuit according to the shooting mode set at the mode setting circuit.
According a third aspect of the present invention, there is provided a distance-measuring method for a camera comprising: a first step of measuring a subject distance on the basis of a subject image signal produced by receiving the light from subjects; a second step of measuring the subject distance on the basis of an image signal obtained by removing the steady-state light component from the subject image signal, while projecting light onto the subjects; a third step of causing the second step to operate for a specific time and, according to the image signal obtained from that operation, selecting a subject whose distance is to be measured; a fourth step of setting the operation mode of the camera; and a fifth step of setting an upper limit on the number of subjects selected at the third step according to the set operation mode.
According to a fourth aspect of the present invention, there is provided a distance-measuring method for a camera comprising: a first step of projecting light onto subjects; a second step of receiving the light from the subjects; a third step of setting a distance-measuring area according to the output by the reception of light; a fourth step of setting more than one shooting mode; and a fifth step of controlling the number of distance-measuring areas set at the third step according to the shooting mode set at the fourth step.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.